This invention relates to the production of pigmentary ferrites; more particularly, it relates to an improvement in the calcination step in the production of these ferrites. Pigmentary ferrites are a new class of iron oxide pigments, in which iron oxide is reacted with other metallic oxides to form a series of chemical compounds known as ferrites. For example, zinc ferrite theoretically contains 33.7% zinc oxide, but in practice from about 32% to about 34%, with the remainder being ferric oxide. Magnesium ferrite contains about 20% magnesium oxide. These ferrites, which have a tan color, have a typical formula designated as M(Fe0.sub.2).sub.2, M0.Fe.sub.2 O.sub.3, or MFe.sub.2 O.sub.4. These ferrites are commercially useful as pigments. Other ferrites are useful for magnetic applications.
Except for red iron oxide pigments, other iron oxide pigments, such as those having yellow, brown, or black colors, are subject to color changes when heated above about 200.degree. C. and are subject, therefore, to certain limitations in use. On the other hand, the tan oxides, known as "ferrite tans" or "tans", have the property of retaining their colors when the materials are heated to temperatures in excess of 200.degree. C.
A typical procedure in the preparation of tan oxide includes the steps of mixing a synthetic ferric oxide (or ferric oxide hydrate) with a metal oxide, such as zinc oxide or magnesium oxide, and calcining the intimate mixture. Calcination temperatures of about 925.degree.-980.degree. C. (1700.degree.-1800.degree. F.) are needed to react the metal oxide and the iron oxide to form the compound known as ferrite. It is during this calcination step that problems arise. When using a rotary calciner, material cakes on the walls, leading to inefficient heat transfer and "overburning" of the material. Hammers or vibrators are needed to remove the caked material from the calciner walls, and these broken pieces of hard material are difficult to mill to give a proper particle size. When a screw conveyor calciner is used, material cakes on the screw flights and becomes hard and "overburned". The build-up of this caked material may be such that the screws jam, or a piece of the hard, caked material can break off, jamming the screws. A build-up of cake is noted by an increase of torque in the screw drive mechanism. Jams may occur several times during an eight hour working shift. When the screw conveyors jam, the screw flights are exposed to the high heat of the calciner, leading to damaged screws. Furthermore, the smaller pieces of caked material that may pass through the screw conveyors are hard and are thus difficult to mill to the proper particle size.